There is growing demand for the rechargeable aprotic Li-air (or Li-O₂) batteries with high energy density for use in portable electronic devices and electric vehicles, as the performance of Li-ion batteries approaches the theoretical limit of energy density. For an aprotic Li- O₂ battery, an insoluble film of lithium peroxide (Li₂O₂) is formed at the positive electrode during the discharge process, which is electrochemically decomposed with the evolution of O₂ gas during the charge process. As the overpotentials for the positive electrode reactions are high, various electrocatalysts, including noble metals, metal oxides, metal nitrides, and heat-treated metal complexes, have been developed. Although such electrocatalysts deposited on electrode substrates have improved the battery performance during the initial stage of reaction, the catalytic activity gradually declines as the electrocatalysts become buried in the Li₂O₂formed over time during the discharge process.

One approach to overcome this inherent problem with electrocatalysts is to use a soluble catalyst that can repeatedly adsorb on the growing or dissolving Li₂O₂ front. However, the solubility of the catalyst used so far was less than several millimoles per liter, which would prevent further improvement of the charging performance. We have recently reported in a three-electrode system that metal complexes with nitrogen-containing macrocyclic ligands, such as metal porphyrin and metal phthalocyanine, can generally catalyze the aprotic OER. Although cobalt phthalocyanine adsorbed on carbon substrate was known to serve as an adsorbed electrocatalyst, our previous study revealed that such metal complexes could function also as soluble catalysts. In addition, we also revealed that the onset potential of the OER was determined by the redox potential of the metal complexes. These results suggest that a metal complex with both a more negative redox potential and with high solubility would function as a good catalyst for Li-O₂ batteries. Herein, we demonstrate that tb-CoPc could serve as an OER catalyst that exhibits superior charging performance for Li-O₂batteries.

The effect of tb-CoPc addition on the discharge/charge characteristics was investigated using a coin type cell. Although both cells showed a stable discharge potential at around 2.7 V, there is a clear difference between the charging profiles. For the cell without tb-CoPc, the charging potential was around 4.0 V and then finally reached ca. 4.3 V (black line in Figure). In contrast, the cell with 10 mM tb-CoPc exhibited better charging performance (red line in Figure). To confirm the effect of the high solubility of tb-CoPc, the effect of CoPc addition, which has lower solubility than tb-CoPc due to the lack of tert-butyl-groups, was investigated. The solubility of CoPc in the representative Li-ion electrolyte that was used (i.e., 1 M Li trifluoromethanesulfonate dissolved in triethylene glycol dimethyl ether (TEGDME)) was not more than 1 mM. Notably, the redox potential of tb-CoPc is more negative than that of CoPc, which is also advantageous for the charging process. However, the charging performance was not so improved when the concentration of tb-CoPc is 1 mM, indicating that the improved charging performance in the presence of 10 mM tb-CoPc is mainly due to the higher solubility of tb-CoPc compared with CoPc. Next, the formation and decomposition of Li₂O₂ in the presence of tb-CoPc was investigated. X-ray diffraction (XRD) analyses revealed that the dominant product at the positive electrode surface from the discharge process was crystallized Li₂O₂. After the charge process, the XRD peaks attributed to Li₂O₂ disappeared, which indicated that Li₂O₂was decomposed during the charging process. 

In conclusions, we have demonstrated that tb-CoPc functions as a soluble catalyst on the positive electrode for rechargeable Li-O₂ batteries. The superior discharge/charge processes accompanied with the reversible formation and decomposition of Li₂O₂ indicates that tb-CoPc functions as an OER catalyst without changing the reaction scheme. We speculate that the soluble tb-CoPc repeatedly adsorbs on the growing and dissolving fronts of Li₂O₂and catalyze (or mediate) the positive electrode reactions. The results obtained here reveal the importance of an appropriate design for the soluble catalyst in rechargeable Li-air batteries to achieve high round-trip energy efficiency.

References

[1] S. Matsuda, S. Mori, K. Hashimoto, and S. Nakanishi, J. Phys. Chem.C., 2014, 118 (49), 28435.

[2] S. Matsuda, S. Mori, Y. Kubo, K. Uosaki, K. Hashimoto, and S. Nakanishi, Chem. Phys. 2015, 620, 78.